The present invention relates generally to a digital impedance measurement system and, in particular, to a digital eddy current system used to facilitate reducing the sensitivity of an eddy current transducer to a magnetic field on a target.
At least some known eddy current proximity systems which analyze and monitor rotating and reciprocating machinery include signal conditioning circuitry and a proximity or eddy current transducer that is positioned proximate a target object, which may include, but is not limited to, a rotating shaft of the machine, or an outer race of a rolling element bearing. The proximity transducer may be a noncontacting device which measures displacement motion and the position of an observed conductive target material relative to the transducer. The target, proximity transducer, and conditioning circuitry components may interact such that a voltage output from the circuitry is directly proportional to a distance or “gap” between the transducer and the target.
The conditioning circuitry measures the electrical impedance (Zp) of the electrical combination of the target, the transducer including an integral sensing coil and cable, and the conditioning circuitry. The impedance is linearized and converted into a voltage that is directly proportional to the gap. The impedance is measured at a specific frequency that is a function of the conditioning circuitry. A target that is magnetized, either due to a residual magnetism or due to an induced magnetism, may cause the measured impedance to be inaccurate and unpredictable. For example, a magnetic field may be induced into the shaft of a rotating electrical machine during normal operation. However, because the magnetic field may be induced non-uniformly around the periphery of the shaft in an area where the proximity transducer may be monitoring, as the shaft rotates, the non-uniformity of the magnetic field may adversely affect the impedance value, which may in turn affect the output gap value.
As a result, at least some known proximity systems may include a calibration procedure to compensate for the effects of a uniform magnetic field. However, magnetic fields affecting targets are often non-uniform, making field calibration procedures impracticable.